This invention relates to the field of semiconductor integrated circuits and, particularly to stand-alone and embedded memory chips fabricated using Silicon-on-Insulator (SOI) substrates having partially depleted (PD) and fully depleted (FD) devices fabricated on the same chip.
Silicon-on-Insulator (SOI) technology employs a layer of semiconductor material formed over an insulating layer on a supporting bulk wafer. The structure can be formed by different well-known techniques in the art, for example, separation by implanted oxygen (SIMOX), bonding and etching back (BESOI), and zone melting and re-crystallization (ZMR), among others. Typically, the structure comprises a film of monocrystalline silicon formed over a buried layer of silicon oxide, which is formed over a monocrystalline silicon substrate.
SOI substrates are being used to manufacture everything from microprocessors to SRAMs. SOI substrates offer increased drive current, lower parasitic capacitance, improved sub-threshold behavior, and improved packing density for integrated circuit processing. These qualities of SOI technology combine to provide distinct speed advantages for devices utilizing such substrates.
DRAM memory circuits are currently the most popular type of memory circuits used as the main memory of processor-based systems. Efforts have been made to apply SOI technology to DRAM chips. However, because of the floating body effects present in partially depleted SOI devices, widespread application has been impractical due to the negative impact on access device performance caused by these effects.
Floating body effects are caused by a build up of charge in the silicon region under the channel depletion region. This charge build up alters the I/V curve causing xe2x80x9ckinksxe2x80x9d or sharp irregularities in the current-voltage curve, lowers threshold voltage (Vt), and causes transistor performance to have a history dependence. The body effect can cause serious degradation to SOI transistor performance in certain applications. Due to this degradation, DRAM circuits have largely been limited to being fabricated on fully depleted SOI substrates where the depletion region under the gate extends to the insulating buried oxide (BOX). Despite the discussed drawbacks, in some circumstances the floating body of partially depleted SOI devices may provide certain advantages over fully depleted devices. For example, a partially depleted device may provide higher drive current through the channel region, which will allow for faster operation of the integrated circuit. This characteristic of partially depleted devices is useful in the periphery devices of a DRAM chip because of their need for increased operation speed and drive current.
There is a need for a simplified method of forming a memory circuit on a SOI substrate where the transistor devices may be formed over both partially depleted and fully depleted regions so that the advantages of each transistor type, and the advantages of the SOI substrate, may be utilized in combination in a single memory chip. A memory circuit formed by such a method would achieve increased drive current by incorporating the partially depleted devices as discussed above, thereby allowing the IC to run faster and more efficiently. It would also achieve the advantages of the improved device behavior and refresh characteristics available to fully depleted SOI devices. It would be optimal if such a method for forming both fully depleted and partially depleted devices on a single SOI substrate could utilize the currently available techniques for fabricating a semiconductor device and limit the necessary steps for forming such a device to as few as possible, thereby saving time and costs.
This invention provides a simple method for forming both partially depleted (PD) and fully depleted (FD) devices on a single memory chip. By utilizing the process of this invention, memory chips may be obtained that utilize both the device behavior advantages of fully depleted devices and the drive and speed advantages of partially depleted devices. Additionally, by utilizing the process of this invention, Silicon-on-Insulator (SOI) substrates may be used so as to obtain the performance advantages of such a dual-depletion region substrate.
Additionally, this invention utilizes common process steps used in current DRAM manufacturing. The dual-depletion regions may be formed simultaneously, thereby reducing the number of steps and required time of processing. Additionally, multiple steps may be performed using a single mask, resulting in a self-aligned process. The process of this invention results in a simple dual-depletion region SOI structure that is less expensive to manufacture.
The above-described and other advantages and features of the invention will be more clearly understood from the following detailed description of an exemplary embodiment, which is provided in connection with the accompanying drawings.